https://leetcode.cn/problems/remove-all-adjacent-duplicates-in-string/description/
class Solution {
public:
    string removeDuplicates(string s) {
        string ret;
        for(const auto&ch:s)
        {
            if(ret.size()&&ch==ret.back())
                ret.pop_back();
            else
                ret.push_back(ch);
        }
        return ret;
    }
};